An ignition system using electric discharge has been widely used for igniting an injected fuel in engines. In such an ignition system, a DC voltage is generated by boosting voltage of a storage battery, and is then further boosted to have a higher voltage value using a flyback transformer. The boosted voltage is guided to a spark plug and discharged, thereby to ignite the fuel.
In this case, when a high DC voltage can be obtained by boosting the voltage of the storage battery, a boost ratio of the flyback transformer can be set to a small value and thus a ratio of number of turns of a winding can be reduced. As a result, the number of turns of a secondary winding can be reduced, and the size of the flyback transformer can be reduced. Also, a conduction current of a primary winding can be reduced. Therefore, conduction loss of the flyback transformer can be reduced, resulting in an increase in efficiency.
A general boosting chopper circuit is made of a series circuit of an inductor and a switching element connected between input terminals, a capacitor connected between output terminals, and a diode that guides a current flowing in the inductor to an output side when the switching element is turned off. When a high boosting ratio is tried to obtain using the boosting chopper circuit, power loss is likely to increase, resulting in decrease in the efficiency.
For example, a non-patent literature 1 discloses a DC-DC converter that realizes a high boosting ratio. In the DC-DC converter, two systems of series circuits each made of an inductor and a switching element are provided between input terminals on a low-voltage side, and N-stages of voltage multiplier cells each made of a capacitor and a diode are provided between output terminals on a high-voltage side.
A connection node of the inductor and the switching element of one of the series circuits is directly connected to the voltage multiplier cell on the lowest stage. A connection node of the inductor and the switching element of the other of the series circuits is connected to the voltage multiplier cell on the lowest stage through an auxiliary capacitor.
In such a structure, a connection node between the auxiliary capacitor and the voltage multiplier cell serves as one of the output terminals, and a terminal of the voltage multiplier cell on a highest stage serves as the other of the output terminals. In the converter, because the two switches are interleaved according to drive signals having a phase difference of 180°, current ripples of the two inductors are cancelled. Therefore, inductance of the two inductors can be reduced, as compared with the general boosting chopper circuit mentioned above. As a result, the number of turns of the winding reduces, and the loss of the inductors reduces.
Further, the switch is applied with a low voltage, which has a value half of the value obtained by dividing an output voltage by the number of stages of the voltage multiplier cells at most. Therefore, the switching loss reduces, as compared with the general boosting chopper circuit mentioned above.
Non-patent Literature 1: P. Kim, S. Lee, J. Park, and S Choi, “High Step-up Interleaved Boost Converters Using Voltage Multiplier Cells”, 8th International Conference on Power Electronics, pp. 2844-2851 (2011)